Abstract: Aiming at the importance of microstructure on the properties of the weldment, the characteristic of the microstructure of laser welded joint in different regions were studied. The differences of weld forming under different laser welding parameters were analyzed, and a magneto-optical imaging method for researching weld microstructure was proposed. The color, brightness and gray value of magneto-optical image of the weld show the qualities of the weld forming under the constant magnetic field. Laser welding test consists of four groups with butt welding of low carbon steel. The magneto-optical images of the weld microstructure and the scanning electron microscope images were combined to analyze the difference of the microstructures, the grain compositions and characteristics of the weld joints in different regions. The experimental results show that the weld joint can be divided into the fusion zone, the heat affected zone and the base metal zone in magneto-optical images, and the grain's compositions, sizes and shapes in the three regions are obviously different.